The invention relates a method for controlling (open loop or closed loop) the level of a vehicle body of a motor vehicle by means of a level control system having actuators with which the level of the vehicle body can be lowered by discharging an actuating means from the actuating elements. The level control system also includes a control unit which carries out the following method steps: determining the level of the vehicle body relative to a reference point; and, initiating a discharge operation of the vehicle body when the determined level lies above a pregiven level.
Modern motor vehicles often have a level control system, for example, in the form of an air-spring system which has actuators in the form of air springs. A pregiven level is stored in the control unit of the level control system. The vehicle body is intended to assume this level compared to a fixed reference point. The pregiven level can, if required, be adapted by the driver of the motor vehicle to the actual driving situation. If, for example, an especially large ground clearance of the motor vehicle is needed offroad, then the driver can, for example, pregive a correspondingly high level in the control unit via a switch on the dashboard.
Level control systems afford the advantage that the pregiven level can be maintained independently of the loading condition of the motor vehicle by lowering or lifting the vehicle body. Furthermore, the vehicle body can also be lowered to a low level, for example, in order to facilitate a simple loading of the vehicle.
However, in a motor vehicle having a level control system, the following problems can occur. The vehicle body of the motor vehicle can seat on the ground, for example, on snow or on the ground when the vehicle is offroad. The wheels of the motor vehicle then drop away relative to the vehicle body. As a consequence of this situation, the actual level of the vehicle body can lie higher than the level, which is pregiven by the control unit, so that the control unit initiates a discharge operation of the vehicle (for example, by evacuating pressurized air out of the air springs of an air-spring system) in order to compensate the actual level to the pregiven level. This process increases the seating of the vehicle on the ground so that an unhindered further travel of the vehicle is not possible.
Another problem results when the motor vehicle is raised by means of a lift. In this case, the wheels of the vehicle deflect completely downwardly so that the vehicle body assumes a maximum level clearly above the level pregiven by the control unit. The control unit then initiates a discharge operation via which the actual level of the vehicle body is intended to adapt to the pregiven level. This takes place in that an actuating means (hydraulic liquid is evacuated in a hydraulic level control system and pressurized air is released from air springs in a pneumatic level control system) is released from the actuators of the level control system. Since the vehicle body is not lowered, the actuators are substantially emptied until only a minimum of the actuating means is held in the actuators. When reaching the minimum, a further discharge of the actuating means is prevented by a valve. When the motor vehicle is taken from the lift, the vehicle body lowers below the pregiven level because of the low level of actuating means in the actuators so that the actuators must again be filled with the actuating means in order to lift the vehicle body to the pregiven level. This is time-intensive and furthermore loads the components of the level control system with which the actuating means is conducted into the actuators.
The same problem results for a single actuator of the level control system when the wheel, which is assigned to this actuator, is lifted with a jack.
In summary, it can be seen that in specific critical situations of the motor vehicle (namely, when the vehicle body is seated on the ground or is on a lift) a lowering of the vehicle body or an emptying of the actuators of the level control system is initiated by the control unit even though this is not wanted in these situations.
It is an object of the invention to provide a method for controlling the level of a vehicle body of a motor vehicle by means of a level control system wherein a seating of the vehicle body on the ground or on a lift is detected as early as possible in order to avoid an unwanted lowering of the vehicle level.
The method of the invention is for controlling the level of a vehicle body of a motor vehicle with a level control system having a plurality of actuator elements with which the level of the vehicle body can be lowered by discharging an actuating means from the actuator elements. The level control system includes a control unit and the method includes the steps of: determining a first level of the vehicle body relative to a reference point directly before a discharge operation; initiating a discharge operation of the vehicle body when the first level lies above a pregiven level; after initiating a discharge operation, checking in the control unit as to whether the vehicle body has lowered from the first level in accordance with an input; and, interrupting the discharge operation when the vehicle body has not lowered from the first level in accordance with the input.
A discharge operation is understood to mean an operation wherein the current level of the vehicle body is intended to be adapted to a level, which is pregiven in the control unit, by discharging actuating means from the actuators of the level control system until this pregiven level is reached. Correspondingly, the interruption of a discharge operation is understood to be a prevention of a further discharge of actuating means from the actuators of the level control system even though the current level of the vehicle body is above the level pregiven in the control unit. The current level of the vehicle body is preferably determined by one or several elevation sensors.
The basic idea of the invention is that, after initiating a discharge operation via the control unit, a check is made after a short time span as to whether the vehicle body has lowered from a determined level in accordance with an input and that the control unit interrupts the discharge operation when this is not the case.
The advantages achieved with the invention are especially seen in that a discharge operation is interrupted early by the control unit when this control unit determines that the vehicle body of the motor vehicle has not lowered from the level, which was determined before the discharge operation, in accordance with an input, because, for example, the vehicle body of the motor vehicle is seated on the ground or is raised by a lift or a vehicle is on the lift with the wheels free. A vehicle body which is seated on the ground is therefore not lowered further or lowered further only minimally and a xe2x80x9cmore intense landingxe2x80x9d of the vehicle body is thereby prevented.
A further advantage of the invention is seen in that a touchdown of the vehicle body on the ground or on the lift is automatically detected by the control unit and is interrupted. An active intervention of an operator is thereby not necessary. A further advantage of the invention is that, to execute the method of the invention, only components are necessary which are anyway present in level control systems so that the cost of the level control system is not increased notwithstanding the added comfort and safety provided.
According to another feature of the invention, the control unit interrupts the discharge operation even when the vehicle body is lowered in accordance with an input after initiation of the discharge operation but, after initiating the discharge operation, the actuating means pressure or at least a wheel contact force has been reduced beyond a pregiven amount. Based on the reduction of the actuating means pressure or the wheel standup force, the control unit determines that the vehicle has touched down, for example, on the ground, during the discharge operation. The advantage of this embodiment is that, in this case, the discharge operation is interrupted so that a harder touchdown of the vehicle on the ground is prevented.
According to another feature of the invention, only a small amount of the actuating means is discharged from the actuator after initiating a discharge operation. Preferably, the quantity of the discharged actuating means is significantly less than the quantity of the actuating means which one would have had to discharge from the actuator in order to lower the vehicle body to the pregiven level from the level determined in advance of the discharge operation. The advantage of this embodiment is that the quantity of the actuating means in the actuator is substantially retained when the check is made in the control unit as to whether the vehicle body has lowered in accordance with an input. The foregoing notwithstanding, the control unit decides whether the vehicle body seats on the ground or on a lift. If the vehicle is on a lift and is taken down from there, then it does not sink or only insignificantly compared to the level which it had before being lifted by the lift so that no or only a slight correction of the level of the vehicle body is required. A longer lifting operation thereby becomes unnecessary (in contrast, in conventional level control systems, the actuators are substantially emptied so that a longer lifting operation of the vehicle body is necessary when the vehicle is taken from the lift).
According to another feature of the invention, a plausibility consideration is executed in advance of the initiation of the discharge operation as to whether the initiation of the discharge operation can lead to a lowering of the vehicle body. A discharge operation is only initiated when this is the case. The plausibility consideration can, for example, be carried out with the aid of force sensors introduced in the tires of the motor vehicle. The wheel contact forces are determined with the aid of these sensors. When the force sensors determine a low or no wheel contact force in advance of initiating the discharge operation, then this is an indication that one or several wheels of the motor vehicle hang free in air and that therefore the motor vehicle is disposed, for example, on a lift. The initiation of the discharge operation can then not lead to a lowering of the vehicle body and is thereby not carried out. Alternatively, a plausibility consideration can be carried out in a pneumatic level control system by checking the air pressure in the air springs of the system, for example, with the aid of pressure sensors. If the air pressure in the air springs is so low that the vehicle level, which is determined in advance of the discharge operation, cannot be assumed at this air pressure even for an omitted or slight loading of the motor vehicle, this is then an indication that one or several wheels of the motor vehicle hang free in the air. The initiation of the discharge operation can in this case too not lead to a lowering of the vehicle body and therefore it is not carried out.
According to another feature of the invention, and after initiating the discharge operation, a check takes place in time intervals as to whether the vehicle body has been lowered during a time interval and the control unit interrupts the discharge operation when this is not the case. The advantage of this embodiment is that even after a short time span (typically, the time intervals have a length of 1 to 5 seconds) it can be determined whether the vehicle body has lowered at all during a time interval or has even possibly been lifted. With the aid of this feature of the invention, it can be determined rapidly during a discharge operation that the vehicle is raised with a jack or a lift. With this embodiment, the advantage is achieved that a maximum possible quantity of actuating means remains in the actuators of the level control system.
According to another embodiment, the control unit interrupts the discharge operation when, after a time span after which the vehicle body would have had to lower from the maximum level even with a minimum load, the vehicle body has not lowered from the first value.
The advantage of this embodiment is understandable when one considers that, in a level control system, in which the level of the vehicle body is lowered by discharging an actuating means from actuators, the relationship between the quantity of the actuating means in the actuators and the level of the vehicle body is not clear at the maximum level of the vehicle body. For example, the case can occur that a loaded vehicle is unloaded and, because of the unloading, the vehicle deflects into a built-in stop and so moves into the maximum level. If actuating means is discharged from the actuator in this case, then the level of the vehicle body does not sink directly out of the maximum level after initiation of the discharge operation. Instead, the vehicle body only sinks after a certain time span when so much actuating means has been discharged from the actuators that the vehicle body sinks out of the maximum level for a low vehicle weight.
In contrast, this problem is not present for a level which lies below the maximum level and above a pregiven desired level, so that the vehicle body immediately sinks after initiating the discharge operation (except when the vehicle is unloaded or lifted during the discharge operation). With this embodiment, the advantage is achieved that the discharge operation is carried out over at least a time span which is necessary in order to clearly determine whether the vehicle body would have had to sink from the actual level if the vehicle body is disposed at or close to the maximum level in advance of the discharge operation. If the vehicle body is below the maximum level, then a judgement can be made directly after initiating the discharge operation as to whether the vehicle body has lowered. Accordingly, the maximum possible quantity of actuating means remains in the actuators of the level control system at all possible levels of the vehicle body.
According to another embodiment, after a time span, a check is made in time intervals as to whether, during one of the time intervals, the vehicle body has lowered at least by a pregiven amount from the first level and whether the wheel contact forces have remained at least almost constant. The control unit interrupts the discharge operation when this is not the case.
Use can be made of the above embodiment when the level actually determined in advance of the discharge operation lies below the maximum level. In this case, the vehicle body lowers in each time interval with unchanging wheel contact forces already by a pregiven amount except if the vehicle is in a critical situation or is unloaded. The advantage of this embodiment is that a critical situation can be distinguished in the control unit from an unloading of the vehicle during a discharge operation.
According to another embodiment the discharge operation is immediately interrupted when the control unit determines that the actual value of a predetermined vehicle parameter is less than a pregiven value for this vehicle parameter.
The advantage of this embodiment is understandable when one considers the following: the fact that the vehicle body has not, during a discharge operation, lowered in accordance with an input from the level determined in advance of the discharge operation cannot perforce be attributed to the fact that the vehicle is in a critical situation. In a few cases, the fact that the vehicle body has not lowered in accordance with the input, for example, can be attributed to the fact that the vehicle is unloaded during the discharge operation and that the vehicle body has lifted for this reason. If the pregiven value for the above-mentioned vehicle parameter is so selected that it is achieved for a motor vehicle without additional loading and for all additional loadings is exceeded, then a drop below this pregiven value of the vehicle parameter can only mean that the vehicle is in a critical situation. The advantage of this embodiment is that a critical situation of the vehicle is detected very rapidly (because there is a drop below the vehicle parameter) and the unwanted discharge operation can be immediately interrupted.
According to another feature of the invention, the wheel contact force is selected as a vehicle parameter. This wheel contact force assumes a minimum value when the motor vehicle is loaded minimally or not loaded at all. If there is a drop below this minimum wheel contact force, then this is an indication that the vehicle must be in a critical situation. Because only then it is possible that there is a drop below the minimum wheel contact force. The wheel contact force can, for example, be measured by force sensors in the tires which transmit their measurement values to the control unit of the level control system by means of a transmitter. The measured values are evaluated in the control unit.
According to another feature of the invention, the level control system includes at least one air spring with which the vehicle body can be suspended and air pressure is discharged from the air spring during the discharge operation.
According to a further embodiment, air is discharged from the air spring maximally over a time span until there is a drop below a minimal air pressure in the air spring which is so pregiven that the vehicle body, even with minimum load, would have had to lower from the first level in accordance with the input. During the time span, a check is made as to whether the vehicle body has lowered from the first level in accordance with the input. The control unit interrupts the discharge operation when this is not the case up to the end of the time span.
In this embodiment, the vehicle parameter is checked after the elapse of a time span, that is, the air pressure in the air springs of the level control system is evaluated. When, after the time span, the air pressure is so low that there is a drop even below the above minimum air pressure and the motor vehicle does not lower in accordance with the input, then this is an indication that the vehicle is in a critical situation.
According to another embodiment of the invention, the maximum time span over which the air is discharged from the air springs is so selected that the air pressure in the air spring drops within the time span from a maximum air pressure to below the air pressure at minimum loading (unloaded vehicle weight). The maximum air pressure is that air pressure at which the vehicle body assumes the level determined also for maximum additional loading directly in advance of the discharge operation. The advantage is that the method can be carried out without a pressure sensor. Thus, the maximum air pressure (at which the vehicle body, also at maximum load, assumes the level determined directly in advance of the discharge operation) can be determined from a characteristic field. The same applies for the minimum air pressure at minimum additional load. The time span is determined from the most unfavorable inputs which correspond to the time span which is necessary in order to reduce the pressure (which is present at maximum loading) to a pressure level for a minimum loading. Since these pressure values are fixedly pregiven for a series manufactured vehicle, the maximum time duration for the particular level can be determined with the aid of the characteristic fields or a mathematical model formation.
According to another embodiment, the air pressure in the air spring is determined directly in advance of the discharge operation. This is followed by the step of determining over which time span pressurized air needs to be discharged from the air spring in order to drop below the minimum air pressure starting from the previously determined air pressure. The pressurized air is discharged from the air spring over maximally the predetermined time span.
Here too, the discharge operation is interrupted in advance of the elapse of the maximum time span when the vehicle body has reached the desired level. Preferably, the air pressure in the air spring is determined directly in advance of the discharge operation with the aid of a pressure sensor of the level control system. The above-mentioned time span, during which the pressurized air may be discharged from the air spring, can, for example, be determined from a characteristic field. The advantage of this embodiment is that the time span, during which the discharge operation is carried out, can be minimized. For this reason, the maximum possible quantity of pressurized air remains in the air springs of the level control system.
According to another embodiment of the invention, the method can only be carried out when the speed of the vehicle has dropped below a pregiven value. This embodiment is based on the idea that a vehicle typically is only in a critical situation when it has a low speed. Preferably, the method is only carried out when the speed of the vehicle is below 5 Km/hr. The advantage of this embodiment is that the method is only carried out when the vehicle, because of its low speed, can be in a critical situation. Discharge operations above the limit speed are carried out without interruption.
According to another embodiment of the invention, after an interruption of the discharge operation for a switched-off engine of the motor vehicle, a level change of the vehicle body (for example, by the actuation of a key) can be initiated by a person only after the engine is switched on. The advantage of this embodiment is that, when the vehicle is lifted with the aid of a jack or a lift, it is prevented that manually a level change of the system is pregiven because of lack of knowledge or carelessness and the vehicle would release itself from the hold of the jack or the lift.
According to another feature of the invention, after an interruption of the discharge operation while the engine of the vehicle is switched on, a lifting operation of the vehicle can be initiated temporarily and exclusively. The interruption of the discharge operation takes place in most cases when the engine is switched on and when the vehicle sits on the ground. With this embodiment, the advantage is accordingly achieved that an unintentional further lowering and therefore a harder seating of the vehicle on the ground is prevented.
According to still another embodiment of the invention, a discharge operation of the vehicle body can only then be initiated again when the speed of the vehicle exceeds a limit value or when a renewed switch-on of the ignition takes place after switch-off. Preferably, the limit value for the speed lies at 5 Km/hr. The advantage of this embodiment is that a discharge operation of the vehicle body can only be again initiated when, with substantial probability, the vehicle no longer sits on the ground because of its speed.
According to another embodiment of the invention, the actually determined level is below the maximum level. The advantage of this embodiment is that when the vehicle speed is derived from the drive wheels, no discharge operation is initiated when the vehicle is on a lift with a running motor and rotating drive wheels.